It is generally known that some semiconductor modules have a ceramic substrate or an insulating substrate such as that made of aluminum nitride; a front metal plate and a rear metal plate which are made of material such as pure aluminum and joined on the two surfaces, a front surface and a rear surface, of the substrate, respectively; a semiconductor element joined to the front metal plate; and a heat sink which is connected to the rear metal plate and serves as a heat dissipating apparatus for dissipating heat generated by the semiconductor element. In such semiconductor modules, it is required for the heat dissipating performance of the heat dissipating apparatus to be maintained over a long period of time. In conventional configurations, however, cracking or distortion may occur in the joint portions due to thermal stress resulting from the difference in the coefficient of linear thermal expansion among the ceramic substrate, the metal plates and the heat dissipating apparatus, depending on the conditions for use, and thus the heat dissipating performance may deteriorate.
In order to solve this problem, a semiconductor module has been proposed in Patent Document 1. Regarding the semiconductor module of Patent Document 1, a thermal stress alleviating portion is created in the rear metal plate as a step, a trench or a recess having a predetermined depth so as to alleviate thermal stress. This thermal stress alleviating portion is provided so that the volume ratio of the rear metal plate to the front metal plate is set to 0.6 or less.
In semiconductor modules having the above described configuration, heat emitted from the semiconductor element is conveyed from the front metal plate, to which the semiconductor element is joined, through the ceramic substrate to the rear metal plate in order so that heat is dissipated via the heat dissipating apparatus. Therefore, it is preferable for the area at which the rear metal plate and the heat dissipating apparatus are joined to be as large as possible, in order to improve the heat dissipating performance. As described above, however, thermal stress is generated between the rear metal plate and the heat dissipating apparatus. While it is preferred to form a thermal stress alleviating portion on the rear metal plate to alleviate this thermal stress, as in Patent Document 1, this makes the joint area smaller by the area of the thermal stress alleviating portion. That is, while it is preferred in semiconductor modules to form a thermal stress alleviating portion even if the joint area at which the rear metal plate and the heat dissipating apparatus are joined becomes smaller, the efficiency in heat dissipation is lowered due to the reduction in the area of the heat conveying portion when the joint area is too small. Therefore, it is necessary to maintain good balance between alleviation of thermal stress and increase in the heat dissipating performance.    Patent Document 1: Japanese Unexamined Patent Publication 2003-17627